In a tank in which an easily polymerizable compound is stored, polymerization in the tank has hitherto been prevented by connecting the delivery piping fitted to the tank to the tank by a circulation line and circulating the liquid residing in the tank through the circulation line.
However, when solution supply to the tank is stopped, the solution residing in the acceptance piping connected to the tank does not move. There has hence been a problem that solution polymerization or freezing occurs in this piping to clog the piping.
Furthermore, since the solution residing should be withdrawn in order to avoid the polymerization or freezing, a product loss has been incurred.
On the other hand, in pipings for transporting corrosive liquids including (meth)acrylic acid, carbon steel has generally been used extensively as the material of flanges, bolts, nuts, etc. as members for these pipings from the standpoints of a reduction in construction cost, etc.
However, the carbon steel is gradually corroded by the corrosive liquid during long-term use, and there have been cases where even when the joints are tightened so as to stop leakage of the corrosive liquid, this liquid leakage cannot be eliminated.
In addition, apparatus, pipings, and the like in which such a corrosive liquid is handled are frequently fitted with a heat insulator or cold insulator. Because of this, initial leakage is difficult to find out and there are often cases where leakage is found out only when the flanges, bolts, nuts, and the like have come into a considerably corroded state.
Once those piping members come into such a state, it is impossible to prevent liquid leakage by taking measures such as tightening. In some cases, the leakage of the corrosive liquid spreads, unavoidably leading to a problem of environmental pollution.
Repairing the corroded parts is considerably dangerous and the operation for withdrawing the corrosive liquid has necessitated much time. There has hence been a strong desire for a measure against these.
In processes heretofore in use for producing a (meth)acrylic acid compound known as an easily polymerizable substance by catalytic vapor-phase oxidation, the (meth)acrylic acid compound is separated or purified and the product is obtained generally from the top of the final purification column (distillation column).
This product is sent to a quality verification tank for the purpose of quality verification, subjected to a product inspection, and then stored in a storage tank. In some cases, the product is directly sent to a storage tank and stored therein.
In the case where a (meth)acrylic acid compound which is an easily polymerizable substance is produced and/or purified and the product is stored, it is necessary for regulate/keep the storage tank for storing the product so as to be in a relatively low-temperature state in order to prevent polymerization and avoid the formation of a detonating gas and, in the case of acrylic acid, to inhibit the formation of acrylic acid dimer.
Because of this, the storage tank is equipped with an apparatus for temperature regulation (mainly for maintaining a cooled state). This apparatus attached to the storage tank is a cooling coil disposed inside the tank or a heat exchanger disposed outside the tank. Temperature regulation has been thus conducted.
On the other hand, since the product to be obtained from the top of the purification column in each process is easily polymerizable, distillation is conducted usually under vacuum in order to lower the operation temperature.
The gas from the column top is cooled while passing through a condenser, whereby a condensate as a product is obtained. The temperature of this condensate is generally about 40° C. because of industrial utility conditions.
Subsequently, this condensate as a product is transported to a storage tank and stored therein. For the storage, it is necessary to regulate/keep the condensate in the storage tank so as to have a relatively low temperature not higher than that temperature. Hitherto, the attached apparatus for temperature regulation/keeping has been relatively large and the equipment cost has been high.
Furthermore, since (meth)acrylic acid compounds are exceedingly apt to polymerize, there are cases where polymerization occurs during production, for example, in a distillation column to stop the operation of the whole plant for producing the (meth)acrylic acid compounds. There also are cases where the operation of the whole plant for acrylic acid compound production is stopped because of a legal examination or according to the state of the market.
When plant operation is stopped, the process liquids present in the plant are not discarded because they contain valuable substances, and are generally stored in an off-specification product storage tank for the purpose of recovery. In the next operation, valuable substances are recovered or the process liquids are utilized in the plant.
In the case of producing a (meth)acrylic acid compound which is an easily polymerizable substance, an off-specification liquid generates at the time of plant operation initiation or stopping or when part of the apparatus and devices constituting the plant comes not to work properly. In storing this off-specification liquid, it is necessary to keep the liquid at a relatively low temperature in order to prevent polymerization and avoid the formation of a detonating gas and, in the case of acrylic acid, to inhibit the formation of acrylic acid dimer.
Because of this, the storage tank for an off-specification product is equipped with an apparatus for temperature regulation (mainly for maintaining a cooled state). This apparatus attached to the storage tank is a cooling coil disposed inside the tank or a heat exchanger disposed outside the tank. Thus, temperature regulation is conducted.
However, when plant operation is stopped, for example, the liquids in the respective steps, which have various compositions, flow in a high-temperature state into the off-specification storage tank in large quantities. Hence, cooling becomes insufficient, resulting in the possibility of polymerization of the (meth)acrylic acid compound which is an easily polymerizable substance. In addition, since the off-specification liquid is exposed to a high temperature for a prolonged time period, the rate of reactions in which the water, alcohol, and (meth)acrylic acid compound contained in the off-specification liquid change into high-boiling substances by the Michael addition reaction is increased and high-boiling substances are formed in a large amount.
An example of measures against those problems is to increase the cooling ability of the internal (cooling) coil to be disposed as an attached apparatus in the off-specification storage tank or of the heat exchanger to be disposed outside the tank to such a degree that the high-temperature liquids which generate in large quantities in the plant operation stopping can be sufficiently cooled. Those problems can be thus coped with.
However, to install a large heat exchanger or the like which is effective in plant stopping occurring once or twice a year is economically disadvantageous, and some measures have been desired.